1. Field of the Invention
This invention relates to a semiconductor module having a plurality of circuit boards that are closely stacked and, more particularly, to means for preventing a short-circuit when a plurality of circuit boards each having IC packages on two opposite surfaces are superposed one on another.
2. Description of the Related Art
A type of semiconductor module among those constructed as IC cards or the like is arranged to improve the integration density of IC packages in the module by stacking a plurality of circuit boards. As IC packages mounted on circuits boards in such a semiconductor module, the use of lead-on-chip (LOC) type IC packages each having a structure in which inner leads extend on a chip in the package has been increased. In such LOC type IC packages, inner leads can be extended to a central portion of a semiconductor chip, for example, by using a tape-automated-bonding (TAB) tape or the like, and wire bonding is not performed. Accordingly, this type of package is advantageous in various respects in comparison with the ordinary package in which inner leads extending to the periphery of chip are connected to electrodes on the chip by wire bonding. For example, a larger chip having a large capacity or multiple functions can be packed in one package, restrictions on the disposition of electrode terminals on a semiconductor chip are reduced, and the overall thickness of the package can be reduced. Specifically, in LOC type packages, leads extend to the outside through side surfaces not at a center but at an upper or lower end of the side surfaces, since LOC type packages have a structure having an inner lead layout on a chip. The present invention relates particularly to a semiconductor module in which a plurality of circuit boards each having two surfaces on which IC packages having leads extending from upper or lower end portions of their side surfaces are mounted, the circuit boards being superposed one on another.
FIG. 11 is an exploded perspective view of a conventional IC card 100 shown as an example of this type of conventional semiconductor module. The IC card (semiconductor module) 100 has a circuit board unit 1 formed of two circuit boards 1a, 1b and a flexible connection portion 1c which connects these circuit boards. IC packages 2 and 3 are mounted on two surfaces of each of the circuit board 1a, 1b. Chip parts 4 each consisting of a chip resistor or a chip capacitor are also mounted on the two surfaces of each circuit board. A connector 6 is provided for electrical connection to an external unit (not shown) such as a terminal. The circuit board unit 1 and the connector 6 are accommodated in a frame 7. Protective metallic panels 5 are attached to obverse and reverse surfaces of the frame 7 so as to cover the circuit board unit 1 and the connector 6 on the upper and lower sides of the IC card.
The circuit board unit 1 is formed of two circuit boards 1a, 1b each having two surfaces on which IC packages 2, 3 and other parts are mounted, and connection portion 1c which electrically and mechanically connects these circuit boards. On each of the two surfaces of each circuit board 1a or 1b, Cu patterns (not shown) are formed for circuit wiring thereon. The IC packages 2, 3 and the chip parts mounted thereon are connected to each other by the Cu patterns. Electrical connections between the circuit boards 1a, 1b are made by Cu patterns (not shown) on the connection portion 1c. Electrical connections between the connector 6 and the circuit board unit 1 are made by a plurality of connector leads 6a located between the connector 6 and the circuit board 1b. The circuit board unit 1 to which the connector 6 is connected is accommodated in the frame 7 by being doubly folded at the connection portions 1c so that the circuit boards 1a and 1b are superposed and fixed in the frame 7. Thereafter, the metallic panels 5 are attached to the opposite sides of the frame 7 accommodating the circuit board unit 1 and the connector 6. The material of the panels 5 is, preferably, a metal if protection of the IC packages 2, 3 from external forces and external electrical noise is taken into consideration.
FIG. 12 is a cross-sectional view of a portion of the IC card 100 shown in FIG. 11, showing the two-layer structure of the circuit boards 1a, 1b. As shown in FIG. 12, each of the IC packages 2, 3 mounted on the circuit boards 1a, 1b is of the above-described LOC type. In each IC package 2, a chip 2a is incorporated at a center of a package body 2d, as indicated by the broken line, and inner leads 2b extend over the chip 2a from the opposite sides. Accordingly, outer leads 2c (hereinafter referred to simply as "leads") extend to the outside through upper portions of the side surfaces of the package body 2d (hereinafter referred to as "upper lead type IC package"). Each IC package 3 is substantially the same as the IC package 2 and is mounted by inverting the upper and lower sides. Leads 3c extend from lower portions of side surfaces of a package body 3d and are bent in a direction opposite to the direction in which the leads 2c of the IC package 2 are bent. (Leads 3c are hereinafter referred to as "lower lead type package".) Upper lead type IC packages 2 and lower lead type IC packages 3 are mounted on the two surfaces of each of the circuit boards 1a, 1b by being randomly positioned. Chip parts 4 are also mounted on the circuit boars 1a, 1b.
To increase the density of integration of the circuit boards, the circuit boards are stacked so that substantially no gap is formed therebetween. Accordingly, the IC packages on the lower mount surface of the circuit board 1a and the IC packages on the upper mount surface of the circuit board 1b are maintained close to or in contact with each other in a back-to-back relationship. Also, a predetermined gap is formed between each metallic panel 5 and the IC packages 2, 3 on the outermost mount surface facing the metallic panel 5.
There have also been provided semiconductor modules having other types of structures, for example, one (IC card) in which a circuit board unit having three or more circuit boards connected to each other is accommodated by being folded in a zigzag manner, and one in which a plurality of circuit boards each having IC packages mounted on its two surfaces are closely arranged on a mother board while being set upright thereon and are covered with metallic panels.
In the conventional semiconductor modules, as described above, upper lead type IC packages and lower lead type IC packages are randomly mounted on circuit boards, and it is therefore possible that, in a situation where the circuit boards are warped or the circuit boards or the entire card is deformed by an external force, and where both two IC packages mounted close to each other in a back-to-back relationship by being interposed between two circuit boards are upper lead type IC packages, leads of the IC packages, particularly shoulder portions thereof (such as those indicated at 20 in FIG. 12) will be brought into contact with each other to cause a short-circuit. In the case of IC packages with multiple pins, in particular, errors in the formation of leads are large and some lead shoulder portions may rise excessively. In such a situation, the possibility of contact between the leads is strong. Also, in the case of an upper lead type IC package mounted on a mount surface facing a metallic panel, electro-static discharge (ESD) from the metallic panel may be directly applied to shoulder portions of the leads of the IC package and cause an error in operation or breakdown of the IC package.
Further, in the case of a circuit board unit in which a plurality of circuit boards are connected as shown in FIG. 11, only one of the circuit boards is connected directly to the connector and, therefore, the pattern wiring from the connector to the other circuit boards not directly connected to the connector is excessively long and therefore has such a high line impedance that there is a possibility of failure to maintain a signal potential at a predetermined level.
The conventional semiconductor modules have the above-described drawbacks.